Various types of wells are drilled into subterranean formations in search of water or steam such as in geothermal wells, oil, gas, uranium, and coal. Workover and completion fluids are those fluids used in the completion and servicing of such wells. Drilling muds or drilling fluids, unlike workover and completion fluids, are those fluids used in the drilling of the well. The compositions of workover and completion fluids can differ from the compositions of drilling muds since workover and completion fluids have functions which differ from those of drilling fluids. A workover or completion fluid differs from a drilling fluid in several ways. A workover fluid does not have to be compatible with drilled solids. Usually, a workover fluid is used in cased holes, and the formation is exposed only at perforations. Whereas a drilling fluid must protect the formation from swelling or disintegration, a workover fluid must not damage the permeability of the formation. Ideally, a workover fluid would contain no solids. If solids must be used, they should be acid soluble. Similarly, the compositions, properties and functions of workover fluids can differ from those of completion fluids.
Drilling muds are those fluids used in rotary drilling to cool and lubricate the bit, lift cuttings from the borehole, and to control borehole pressure. Drilling muds should have a viscosity, density, and a fluid retention level suitable for the particular drilling application and the formation being drilled. For instance, a drilling mud generally must be sufficiently dense to control the pressure of the well and simultaneously sufficiently plastic to carry and lift cuttings from the well.
Completion fluids are those fluids used after drilling is complete and during the steps of completion of the well. Completion can include cementing the casing, perforating the casing, setting the tubing and pump, etc.
Workover fluids are those fluids used during remedial work in the well. This can include removing tubing, replacing a pump, cleaning out sand or other deposits, reperforating, etc. Workover also broadly includes steps used in preparing an existing well for secondary or tertiary oil recovery such as polymer additions, micellar flooding, steam injection, etc., or for well stimulation such as acidizing, fracturing the formation, etc.
Both workover and completion fluids are used in part to control well pressure, to prevent the collapse of tubing from overpressure, and to prevent or reduce corrosion of casing. A drilling fluid may be suitable for completion or workover applications in some cases, but not in all cases. When a special workover or completion fluid is used, it is usually a poor or unsuitable drilling fluid.
Although there has been considerable progress in the field of workover and completion fluids, there is significant room for further improvement. For instance, workover and completion fluids which retain their properties at elevated temperatures are in demand.